Automotive fuses commonly comprise a synthetic plastic housing containing an all metal piece stamped from a strip of meltable fuse metal to form relatively rigid terminal blades. When the fuse is assembled in an insulating housing, these terminals project in the same plane from the bottom of the housing. Current-carrying extensions of the terminal blades located in the housing are interconnected within the housing by a relatively fragile fuse link which melts and opens the circuit upon hazardous prolonged overload or short circuit conditions. Such a fuse is disclosed in U.S. Pat. No. 4,635,023, granted on Jan. 6, 1987. The preferred form of the present invention utilizes a housing and terminal blade extensions similar to those disclosed in this patent. In such cases, the present fuse has a resettable, rather than a meltable, fuse link extending between the current-carrying extensions of the terminals. In the preferred form of the invention, the resettable fuse link is a positive temperature coefficient (PTC) material.
Resettable circuit protection devices having a PTC fuse link are now available on the market. They are sold in various different configurations, none of which is like those of the present invention. Some of these prior art resettable circuit protection devices include a PTC element having a plate-like appearance and comprising a thin layer of a PTC material having a pair of thin coatings of metal forming terminals or electrodes on the opposite faces of the PTC layer. A pair of thin wire leads are electrically secured by solder to the opposite conductive faces of the PTC layer.
A variety of PTC elements like that just described are referred to as resettable fuses and are sold under the registered trademark Polyswitch.RTM. by the Raychem Corporation of Menlo Park, Calif. The maximum continuous, non-hazardous current of these Polyswitch.RTM. fuses that will not cause the PTC element at 20.degree. C. to switch from its low to its high resistance state, referred to as the "holding current," presently spans the current range of about 0.9-9.0 amps. The range of trip currents, which is the minimum continuous current that will cause the Polyswitch.RTM. fuses to be switched or tripped to a high resistance circuit-protecting state at 20.degree. C., varies from about 1.8 to 18 amps. This high resistance circuit-protecting state is maintained by a small self-heating current. The largest fault current which such devices can interrupt without being damaged varies from about 50-100 amps. The initial minimum resistance of these circuit protection devices varies from about 0.02 to 0.20 ohms.
The PTC materials believed to be used in these Polyswitch.RTM. fuses are disclosed in U.S. Pat. Nos. 4,237,441, and 4,545,926. These types of PTC materials include a mixture of organic crystalline polymers in which are distributed conductive particles which may include carbon black. With such materials, as current flow therethrough progressively increases they are progressively heated until the current reaches the trip current level. At this level, the resistance suddenly increases to a substantially higher level due to the fact that the material suddenly expands. This expansion separates the conductive particles by larger distances, providing a greatly increased resistance through the particles.
The configurations in which these PTC resettable fuse devices have been commercially available are not suitable for automotive fuse applications.